1. Field of the Invention
The present invention relates to systems and methods of amplifying signals and in particular to a system and method for limiting the average power output of a traveling wave tube amplifier without limiting peak power output.
2. Description of the Related Art
Combining a linearizer with a traveling wave tube amplifier (TWT) allows for more efficient operation of the power amplifier while maintaining linear performance. To fully exploit this advantage it is desirable to optimize the TWT for best efficiency at the operating point. For most TWT designs, optimizing for efficiency at a backed off operating point results in a condition where increasing drive levels above the operating point can be damaging to the TWT due to collector backstreaming and beam defocusing problems. Designing the TWT to handle the higher drive levels also results in increased cost and design complexity.
One possible way of protecting the TWT is to incorporate an input drive limiter into the linearizer. Unfortunately, limiting the drive level is not compatible with linear performance of the system. To provide linear performance under multicarrier operation, an amplifier must have an amplitude modulation-to-amplitude modulation (AMxe2x80x94AM) transfer characteristic that is linear for a range of drive levels above and below the operating point. This requirement is due to the way that the multiple carriers sum together resulting in high peak powers. For example, for two equal carriers, the peak power is 6 dB above the average individual carriers. For multiple carriers, the peak value can be much higher, that is, approximately 8.4 dB for 8 carriers randomly phased. Any limiter would have to be set at a level that is 6 or more dB higher than the operating point and this would eliminate almost all advantages of optimizing the TWT at back-off.
Another way of protecting the TWT is to use limiters that limit both peak as well as average power. These are implemented using a simple saturating amplifier. Such limiters afford some protection to the TWT but in order to preserve linear performance, they must be set at a point that is far above the required operating point. This greatly limits their value in this application. This type of limiter is typically set at a saturation point of the TWT or higher and only provides protection from accidental overdrive conditions.
Although circuits for compensating the gain compression and phase of nonlinear amplifiers such as traveling wave tubes exist in the prior art, none of these circuits include a limiter that limits average power without limiting peak power as well. For example, although U.S. Pat. No. 5,304,944, issued to Copeland et al. discloses a passive limiter made up of PIN diodes, this type of limiter will not achieve the result of the present invention as it will limit peak power as well as average and therefore will degrade linearity if set near the desired operating point. U.S. Pat. No. 5,598,127, issued to Abbiati et al., discloses a procedure and circuit for adjusting the compensation of gain distortions in a microwave amplifier. The procedure is based on a circuit that monitors the ratio of peak to average power and feeds back a signal to adjust the compensation circuit before the amplifier, and in which the control circuit is adjusted such that the peak to average power ratio of the output remains constant. This control circuit provides a means for maintaining linear performance in the presence of changing amplifier linearity due to life or environmental changes, but provides no means of protection for the high power amplifier because the average power is not controlled and could increase beyond the capability of the amplifier. Also, this circuit relies on monitoring the output power of the amplifier, which adds complexity and loss to high power systems.
What is needed is a system and method that allows optimizing a linearized traveling wave tube amplifier for both linearity and efficiency at the operating point while protecting the TWT from inadvertent excursions of input drive levels. The present invention satisfies that need.
As noted above, an optimized TWT design must provide linearity, yet protect the TWT from inadvertent drive level excursions at the same time. The resolution of these apparently incompatible design goals is problem is rooted in the discovery that TWT failure modes and TWT linearity are influenced by distinctly different signal and power dynamics. The present invention advantageously uses a measure of these signal dynamics advantage to provide both TWT linearity and resistance to damage from excessive drive level.
Signal peaks encountered during multicarrier operation exist only for short periods of time and are not damaging to the TWT. The difference between peak and average power becomes more pronounced (and in a predictable manner) as the number of carriers increases. Operating the TWT under multicarrier conditions will not produce damage provided that the average power is not increased above the optimized operating point.
However, the TWT can be damaged by excessive average power levels. These excessive levels generally occur under accidental conditions in test or in operation due to changes in attenuation such as changes in atmospheric attenuation of the uplink signal.
In accordance with the foregoing, the present invention discloses system comprising an average power dependent attenuator and a TWTA, if required a predistortion linearizer can be added to further improve linearity. An attenuator that has a slow frequency response such that peak powers are passed with minimal attenuation but average powers are subject to a large attenuation is positioned before the radio frequency (RF) input of the TWTA. The result is a system that has an AMxe2x80x94AM transfer characteristic that is dependent on average power. For low average power the transfer curve is linear to drive levels far in excess of the required operating point. If the average power is increased to levels above the required operating point the input attenuation changes and the transfer curve shifts to a lower output power but retains its shape. In this way an average output power is maintained that drives the TWTA at the desired operating point and no higher.
The implementation of the average power limiter can be accomplished by a voltage controlled attenuator circuit which is driven by a detector circuit that produces a substantially non-alternating or direct current (DC) voltage proportional to average power.
In accordance with the foregoing, the present invention discloses a method and apparatus for limiting an average power output of an amplifier without limiting the peak power output of the amplifier.
The method comprises the steps of dynamically determining a value proportional to an average power of an amplifier input signal and substantially independent from a peak power of the amplifier input signal, dynamically attenuating the amplifier input signal according to the value, and applying the dynamically attenuated amplifier input signal to an amplifier to produce the amplifier output signal.
The apparatus comprises a detector for dynamically producing a detector signal proportional to an average power of the input signal and substantially independent from a peak power of the input signal, and an attenuator in communication with the detector and the amplifier, for dynamically attenuating the input signal according to the detector signal.
In one embodiment, the detector comprises a current rectifier such as a diode device in series with a low pass filter, which can be implemented by a simple resistive-capacitive (RC) circuit. In another embodiment, the attenuator comprises a shunt limiter such an enhancement field effect transistor (FET) with a gate coupled to the detector.
The foregoing implements an average power limiter that prevents a TWT from operating at output powers higher than the required operating point. Employing this limiter allows for the TWTs to be optimized for performance at the required operating point, resulting in higher efficiency and eliminates the need to size the amplifier for a power level any larger than the required operating point which reduces the cost and complexity of the system. The power limiter protects the TWT while not degrading the linear performance of the TWT by limiting the average power while allowing peak powers of short duration to pass with low loss. This provides an optimized traveling wave tube amplifier optimized for both linearity and efficiency at the operating point while protecting the TWT from inadvertent excursions of input drive levels. The present invention is especially applicable to high power microwave amplifier systems, specifically including systems that operate with multi-carrier signals.